A modest proposal for updateable fields

Details

Description

I'd like to propose a simple design for implementing updateable fields in Lucene. This design has some limitations, so I'm not claiming it will be appropriate for every use case, and it's obvious it has some performance consequences, but at least it's a start...

This proposal uses a concept of "overlays" or "stacked updates", where the original data is not removed but instead it's overlaid with the new data. I propose to reuse as much of the existing APIs as possible, and represent updates as an IndexReader. Updates to documents in a specific segment would be collected in an "overlay" index specific to that segment, i.e. there would be as many overlay indexes as there are segments in the primary index.

A field update would be represented as a new document in the overlay index . The document would consist of just the updated fields, plus a field that records the id in the primary segment of the document affected by the update. These updates would be processed as usual via secondary IndexWriter-s, as many as there are primary segments, so the same analysis chains would be used, the same field types, etc.

On opening a segment with updates the SegmentReader (see also LUCENE-3836) would check for the presence of the "overlay" index, and if so it would open it first (as an AtomicReader? or it would open individual codec format readers? perhaps it should load the whole thing into memory?), and it would construct an in-memory map between the primary's docId-s and the overlay's docId-s. And finally it would wrap the original format readers with "overlay readers", initialized also with the id map.

Now, when consumers of the 4D API would ask for specific data, the "overlay readers" would first re-map the primary's docId to the overlay's docId, and check whether overlay data exists for that docId and this type of data (e.g. postings, stored fields, vectors) and return this data instead of the original. Otherwise they would return the original data.

One obvious performance issue with this appraoch is that the sequential access to primary data would translate into random access to the overlay data. This could be solved by sorting the overlay index so that at least the overlay ids increase monotonically as primary ids do.

Updates to the primary index would be handled as usual, i.e. segment merges, since the segments with updates would pretend to have no overlays) would just work as usual, only the overlay index would have to be deleted once the primary segment is deleted after merge.

Updates to the existing documents that already had some fields updated would be again handled as usual, only underneath they would open an IndexWriter on the overlay index for a specific segment.

That's the broad idea. Feel free to pipe in - I started some coding at the codec level but got stuck using the approach in LUCENE-3836. The approach that uses a modified SegmentReader seems more promising.